As electronic equipment and the associated circuitry become smaller, the pattern widths and spacing of the circuit lines on printed wiring boards have become very narrow. As a result, the copper foil used for making the circuit lines has tended to become thinner, for example, 12 .mu.m thick foils are used, rather than conventional 35 .mu.m and 18 .mu.m foils. Recently, the need for thin copper foil has increased and ultra-thin copper foils have been tried. Handling a copper foil of 12 .mu.m or less, is however, is difficult. For example, it can wrinkle or tear while being produced and/or handled. Similar problems occur when an ultra-thin copper foil is used as the outer layer of a multi-layer printed wiring board. A method of preventing these problems with handling ultra-thin copper foil is needed.
It has been previously proposed to support ultra-thin copper foils on a metal carrier layer which is separated after the copper foil has been laminated to a substrate. Generally, a release layer is used between the copper foil and the metal carrier. Printed wiring boards can be made from such supported ultra-thin copper foils by depositing a copper layer having a thickness of 1-12 .mu.m onto a metal carrier layer having a thickness of 18-70 .mu.m, then applying the exposed surface of the copper layer to a prepreg, such as a glass-reinforced epoxy resin or the like, and laminating by hot pressing. Finally, the metal carrier layer is separated, leaving a copper-clad laminate from which a printed wiring board can be made.
Several carrier metals and types of release layers have been suggested. When aluminum is used as the carrier layer, several types of release layers have been proposed, for example:
1. a release layer of the sulfides or oxides of Cr, Pb, Ni and Ag (for example, in U.S. Pat. No. 3,998,601 and EPO No. 208 177 A); PA1 2. a release layer formed of nickel or nickel alloy plating after an initial zinc plating or zincate treatment (for example, in U.S. Pat. Nos. 3,936,548 and 3,990,926); PA1 3. a release layer of aluminum oxide (for example, in Japanese Patent Application Publication (Examined) No. Sho 60-31915, U.S. Pat. No. 4,293,617, and U.K. Patent No. GB 1,458,260); or PA1 4. a release layer of silica (for example, in U.S. Pat. No. 4,357,395). PA1 (a) cleansing and removing aluminum oxide from the surface of the aluminum carrier layer; PA1 (b) electrodepositing a porous copper layer on the aluminum carrier after step (a); PA1 (c) electrodepositing zinc on the porous copper layer of (b) to form a layer of porous copper with interpenetrating zinc; PA1 (d) electrodepositing a first layer of copper from a bath which does not remove the zinc deposited in (c); and PA1 (e) electrodepositing a second layer of copper sufficient to provide the desired thickness of ultra-thin copper foil.
Such conventional supported copper foils have, however, been found to present problems.
If the release layer is not uniform over the surface of the carrier layer, bond strength between the carrier layer and the ultra-thin copper foil is uneven. Consequently, when the carrier layer is peeled off after laminating a composite foil, if the bond strength is high, some of the ultra-thin copper foil may remain on the carrier layer and the required circuit pattern cannot be made. If the bond strength is weak, the ultra-thin copper foil may separate from the carrier layer prematurely during production and use of the composite foil.
When oxides, sulfides, chromium or inorganic materials, such as chromium or the like, are used as release layers, some of the inorganic material remains on the surface of the ultra-thin copper foil after the carrier layer is peeled off.
Finally, when the composite foil is laminated to a substrate, such as an epoxy prepreg at high temperatures, it becomes difficult to peel off the carrier layer and it may be necessary to use chemical etching to remove the carrier. The high temperatures used in lamination also tend to cause discoloration of the copper foil by oxidation of the surface.
Because of these problems, composites of ultra-thin copper foil on a carrier layer are not generally used in industry at present, despite the methods just discussed.
Accordingly, the present inventors sought a composite foil which overcomes the problems discussed above and a process for making such composite foils. They investigated the metals and/or metal compounds which have conventionally been suggested as release layers for composite foils in the prior art.
The inventors have found that deposition of copper directly on an aluminum surface, or one covered by aluminum oxide, is unsatisfactory since the bond strength is too weak. Consequently, the ultra-thin copper foil may easily be separated from an aluminum carrier during handling or laminating to insulating substrates.
They have found that when zinc is used as a release layer, as suggested by the earlier art, the bond strength is too great to permit easy removal of an aluminum carrier. Thus, it was necessary to etch away the aluminum, with the undesirable consequences of such procedures. It would be preferable to mechanically separate the aluminum carrier and, thus, avoid etching.
Deposition of zinc on aluminum often has been done by a chemical displacement reaction, referred to as a "zincate process." Zinc ions provided by a zinc compound in solution are contacted with the aluminum carrier. The aluminum surface is dissolved and replaced with zinc metal. It is possible to deposit ample zinc metal in a very short time. Thus, the zincate process is convenient, although not easily controlled. Electrodeposition of zinc is more readily controlled. But even with electrodeposition, the adhesion of zinc to aluminum is too strong to permit easy removal of the aluminum carrier.
In another commonly assigned patent application, the problems associated with the bond strength between an ultra-thin copper foil and a metallic carrier have been overcome by the use of certain organic compounds as a release layer. Below, the present inventors will show how zinc can be used successfully between an ultra-thin copper foil and an aluminum carrier.